1. Field of the Invention
The present invention relates to an image sensor.
2. Description of the Related Art
An image sensor comprises photodiodes arranged on a chip. The photodiodes photoelectrically convert optical information from an object into electric image information signals. These image information signals involve noise such as a fixed pattern noise (hereinafter referred to as the FPN).
The image sensor sequentially converts optical data received by a row of the photodiodes (pixels) into voltage levels. When all of the pixels are uniformly irradiated with light, they provide, in theory, an identical output voltage. In practice, however, the outputs of the pixels differ from one another because of the FPN produced by the image sensor.
The extent of the FPN is about several tens of millivolts, although it is dependent on the magnification factor of an amplifier incorporated in the sensor. Since the amplitude of an actual output signal of the image sensor is about 500 mV to 1 V, the FPN can not be ignored, in particular, in a color image sensor involving many intensity levels.
FIG. 1 shows a conventional image sensor to correct the FPN. This image sensor comprises, for each pixel, a precharge transistor 21, a pixel read transistor 22, a photodiode 23, a pixel amplification transistor 24, and a pixel switching transistor 25. The transistors 21 and 22 and the photodiode 23 are connected in series to a precharge line L.sub.PR. One terminal of the pixel amplification transistor 24 is connected to a signal read line L.sub.RE, and the other terminal thereof is grounded through the pixel switching transistor 25. The gate of the pixel amplification transistor 24 is connected to a node between the precharge transistor 21 and the pixel read transistor 22.
FIG. 2, shows a time chart in which pixels to be read of the image sensor are switched from one to another through the pixel switching transistors 25. In a pixel to be read, the precharge transistor 21 is turned ON to charge and initialize the photodiode 23 and set the voltage level of a signal line L1 to be equal to that of the signal read line L.sub.RE, the pixel amplification transistor 24 carries out impedance conversion on a signal from the photodiode 23. The converted signal is transferred to the signal read line L.sub.RE.
The FPN is produced by the transistors 21, 22, 24, and 25 provided for each of the pixels because the characteristics of these transistors vary from pixel to pixel. The FPN fluctuates the output signals of the pixels. The cause of the FPN will be explained more precisely.
DC offsets of the pixel amplification transistor 24 and pixel switching transistor 25 differ from pixel to pixel because of fabrication allowance of the transistors, thereby partly causing the FPN. The precharge transistor 21 causes, when it is OFF, charges other than a signal to flow into the gate of the pixel amplification transistor 24 due to Miller capacitance, thereby partly causing the FPN. For the same reason as the image amplification transistor 24, the pixel read transistor 22 also causes part of the FPN to affect a signal reading when the transistor 22 is turned ON.
To remove the FPN, the conventional image sensor of FIG. 1 employs a correlated double sampling (CDS) circuit 26. As shown in FIG. 2, the waveform of an image signal read involves the FPN. To remove this, the CDS circuit 26 operates as follows:
At first, a signal (.phi..sub.SW)n for activating the pixel switching transistor 25 is set to ON. A signal .phi..sub.PR for activating the precharge transistor 21 is changed from ON to OFF, and a signal (.phi..sub.PH)n for activating the pixel read transistor 22 is changed from OFF to ON. Accordingly, the photodiode 23 and the gate of the pixel amplification transistor 24 are precharged, and a level when the precharge transistor 21 is turned OFF is sampled in response to a rise of a sampling signal .phi..sub.a. This level includes, in addition to the precharged level, DC offset noise of the pixel amplification transistor 24 and pixel switching transistor 25 as well as noise produced when the precharge transistor 21 is turned OFF. Thereafter, the pixel read transistor 22 is turned ON, and a level when a signal from the photodiode 23 is read is sampled in response to a rise of a sampling signal .phi..sub.b. The level sampled with the signal .phi..sub.a is subtracted from the level sampled with the signal .phi..sub.b to cancel the noise components.
The image sensor having the CDS circuit 26 is effective in removing the FPN to some extent. It leaves, however, an FPN of about 30 mV. The inventors of the present invention have found that the FPN is caused when the pixel read transistor 22 is turned ON. It has been generally understood that noise produced when a transistor is turned ON is relatively small. The inventors of the present invention found, however, that it reaches even to about 30 mV. Since the FPN caused by the pixel read transistor 22 is produced at the same timing as providing an output signal, the CDS circuit 26 is theoretically incapable of removing the noise. Namely, other countermeasures for removing the noise must be taken.